Team develops synthetic skin with self-healing properties

Stanford University engineers have developed a synthetic skin capable of sensing subtle pressure and healing itself when torn or cut.

The findings from chemical engineering Prof Zhenan Bao and her team were published on 11 November in Nature Nanotechnology.

In the last decade, there have been major advances in synthetic skin, said Bao, the study’s principal investigator, but even the most effective self-healing materials had major drawbacks.

According to a statement, some had to be exposed to high temperatures, making them impractical for day-to-day use. Others could heal at room temperature, but repairing a cut changed their mechanical or chemical structure, so they could only heal themselves once. Crucially, no self-healing material was a good bulk conductor of electricity, a crucial property.

‘To interface this kind of material with the digital world, ideally you want them to be conductive,’ said Benjamin Chee-Keong Tee, first author of the paper.

The researchers achieved this by combining the self-healing ability of a plastic polymer and the conductivity of a metal.

They started with a plastic consisting of long chains of molecules joined by hydrogen bonds.

‘These dynamic bonds allow the material to self-heal,’ said Chao Wang, a co-first author of the research. The molecules easily break apart, but then when they reconnect, the bonds reorganise themselves and restore the structure of the material after it gets damaged, he said.

To this resilient, bendable polymer, the researchers added tiny particles of nickel, which increased its mechanical strength. The nanoscale surfaces of the nickel particles are rough, which proved important in making the material conductive. Tee compared these surface features to ‘mini-machetes’, with each jutting edge concentrating an electrical field and making it easier for current to flow from one particle to the next.

The result was a polymer with uncommon characteristics. ‘Most plastics are good insulators, but this is an excellent conductor,’ Bao said.

The next step was to see how well the material could restore both its mechanical strength and its electrical conductivity after damage.

The researchers took a thin strip of the material and cut it in half with a scalpel. After gently pressing the pieces together for a few seconds, they found the material gained back 75 per cent of its original strength and electrical conductivity. The material was restored close to 100 per cent in about 30 minutes.

What’s more, the same sample could be cut repeatedly in the same place. After 50 cuts and repairs, a sample withstood bending and stretching just like the original.

The composite nature of the material created a new engineering challenge for the team. Bao and her co-authors found that although nickel was key to making the material strong and conductive, it also got in the way of the healing process, preventing the hydrogen bonds from reconnecting as well as they should.

For future generations of the material, Bao said the team might adjust the size and shape of the nanoparticles, or even the chemical properties of the polymer, to get around this trade-off.